Method and Installation for Producing Light Gauge Steel with a High Manganese Content

ABSTRACT

The production of steels with high contents of manganese (Mn), aluminum (Al), and silicon (Si) with TWIP properties (Twinning-Induced Plasticity) by continuous casting is considered difficult or impossible in the present state of the art for various reasons. These reasons include low strength of the strand shell during solidification due to intense microsegregation of Mn, high strength at relatively low temperatures, reactions of the aluminum in the steel with the flux powder, macrosegregation, depletion of the alloying elements near the surface, and oxidation of the grain boundaries during the reheating of slabs in the pusher furnace. Therefore, in accordance with the invention, successively arranged process steps are used to cast light gauge steel with a given chemical composition of 15-27% Mn, 1-6% Al, 1-6% Si, ≦0.8% C, with the remainder consisting of Fe and accompanying impurity elements on a thin-slab casting machine ( 1 ) (d≦120 mm) with the use of suitable flux powders, to cut slabs ( 3 ) from the endless strand ( 2 ) immediately after solidification, to bring about temperature equalization in a continuous-type intermediate furnace ( 4 ), and then to hot roll the slab ( 3 ) immediately without intermediate cooling.

The invention concerns a method and an installation for producinghot-rolled strip from a highly cold-workable, high-strength, austeniticlight gauge steel with increased contents of manganese (Mn), aluminum(Al), and silicon (Si) with TWIP properties (Twinning-InducedPlasticity), wherein the steel is first cast into an endless strand in acontinuous casting installation, cut into slabs, and then rolled to thefinal thickness.

Austenitic light gauge steels with TWIP properties for use, e.g., inautomobile body parts, reinforcing automobile body members, andcryogenic tanks and pipelines have, for example, according to EP 0 889144 B1, a chemical composition of 10-30% Mn, 1-6% Si, 1-8% Al (withSi+Al≦12%), with the remainder consisting of Fe.

DE 199 00 199 A1 discloses a high-strength light gauge steel with 7-30%Mn, 1-10% Al, 0.7-4% Si, ≦10% Cr, ≦10% Ni, ≦3% Cu, ≦0.5% C, andoptionally with the additional alloying elements N, Va, Nb, Ti, and P,which has not only good mechanical properties but also good resistanceto corrosion and stress-corrosion cracking. This steel is intended to becast by continuous casting and hot rolled or to be cast by thin-stripcasting with near-net shape.

The production of high-manganese steels by continuous casting isconsidered difficult or impossible in the present state of the art forvarious reasons, namely, low strength of the strand shell duringsolidification due to intense microsegregation of Mn (danger of breakoutat Mn>15%), high strength at relatively low temperatures (installationoverloading, cracking problems), reactions of the aluminum in the steelwith the flux powder (limitation of function), macrosegregation,hydrogen and/or oxygen absorption by spray water cooling, increasedoccurrence of nonmetallic inclusions, depletion of the alloying elementsnear the surface, and oxidation of the grain boundaries during thereheating of slabs in the pusher furnace.

In a publication by Spitzer et al.: “Innovative Steel Products—Challengefor Process Development”; Conference Report: Barbara 2001, pp. 71-84, itis stated in this regard that steels with increasing manganese contentsare increasingly difficult to cast. For one thing, they have lowstrength at high temperatures after solidification, since at highmanganese contents, manganese becomes highly concentrated in theresidual melt and lowers the melting point in the interdendriticregions. This increases the tendency towards sticking-type breakouts,which make continuous casting impossible at Mn contents of 15% or moreaccording to present estimates. On the other hand, the steels have highstrength at low temperatures, so that installation overloading occursduring the bending of the steel, and cracking must be expected. Inaddition, at aluminum contents of several percent, which are used inthese steels for the purpose, among others, of reducing the density,reactions occur with the flux powder, which have serious adverse effectson the function of the flux powder.

In another publication by Gigacher et al.: “Properties of High-ManganeseSteels under Conditions Similar to Continuous Casting”; BHM 149 (2004),No. 3, pp. 112-117, it is stated in summary that the casting of thealloying concepts presented there for the production of TWIP steels isnot advantageous for methods with flux powders.

The principal problem with the casting of steels with a high Al content(>1%) is the reaction of the Al from the steel with the oxideconstituents of the flux powder. The reduction of the SiO₂ in the slagby the Al from the steel gives rise to Al₂O₃, which is absorbed by theslag and causes the basicity of the slag (the CaO/SiO₂ ratio) toincrease. This leads to very sharp changes in viscosity and in thelubrication conditions in the mold.

Due to these difficulties, various approaches for the production of TWIPsteels have been taken:

WO 02/101109 A1 discloses a method in which a significant reduction ofthe offset yield stress and thus an improvement of the formability byhot rolling and cold rolling are achieved by increasing the possiblecarbon content (C≦1%) and by adding additional elements, here especiallyB as well as Ni, Cu, N, Nb, Ti, V, and P. To produce this steel, afeedstock (slab, thin slab, or strip) is heated and hot rolled andcoiled within specific temperature limits.

EP 1 341 937 B1 describes a method in which a steel containing 12-30% Mnis cast in a twin-roll casting machine into a near-net shape thin stripwith a thickness of less than 1 mm to 6 mm. The near-net shape stripemerging vertically from the casting gap is cooled by coolants appliedto its surface and is then rolled to the final thickness in a single hotrolling pass. The total time interval between the exit from the castinggap and the entry into the rolling stand is about 8 seconds.

EP 1 067 203 B1 discloses a method for producing strip made of anFe—C—Mn alloy in which a thin steel strip with a thickness of 1.5-10 mmand a composition of 6-30% Mn, 0.001 to 1.6% C, ≦2.5 Si, ≦6% Al, ≦10% Crand other elements is first cast on a twin-roll casting machine and thenhot rolled in one or more stages with 10-60% reduction.

Based on this prior art, the objective of the invention is to specify amethod and an installation which can be realized as simply as possibleand with which high-manganese steels with a given chemical compositioncan be produced by continuous casting.

The objective with respect to the method is achieved by thecharacterizing features of Claim 1, which involve the use of successiveprocess steps, in which a light gauge steel with the given chemicalcomposition of 15-27% Mn, 1-6% Al, 1-6% Si, ≦0.8% C with the remainderconsisting of Fe and accompanying impurity elements

-   -   is cast on a thin-slab casting machine (d≦120 mm) with the use        of suitable flux powders, which very quickly reach equilibrium        and then undergo no further change in their lubrication        behavior, and is then cut into slabs,    -   immediately after the solidification and cutting of a slab,        temperature equalization is brought about in a continuous-type        intermediate furnace, and then    -   the slab is immediately hot rolled without intermediate cooling.

An installation for carrying out the method is characterized by thefeatures of Claim 7.

In the production of thin slabs, for example, on CSP casting machines(CSP=Compact Strip Production), the strand is withdrawn vertically, benthorizontally after solidification, and then cut into slabs. Therefore,no problems can arise with internal cracks. The production ofhigh-strength austenitic steels without installation overloading ispossible and in the meantime has become the state of the art.

Microsegregations that are still present in the strand aftersolidification are largely removed again by diffusion during passagethrough the intermediate furnace, for example, through a roller hearthfurnace, before the subsequent rolling deformation. In the process, themacrosegregations in the slab center are sufficiently equalized duringthe intense deformation in the hot rolling mill, much like high-gradeaustenitic steels.

In accordance with the invention, the use of the roller hearth furnaceof a CSP installation advantageously avoids any relatively greatdepletion of the alloying elements and oxidation of the grain boundariesdue to the short passage time. Depletion of the alloying elements andoxidation of the grain boundaries can cause problems, for example,during the relatively long heating times in the pusher furnace of aconventional hot-rolled wide strip mill in accordance with the priorart.

In accordance with the invention, to be able to use the technology ofcasting TWIP light gauge steels with high Mn and Al contents on athin-slab casting machine, it is necessary to use a suitable fluxpowder. In accordance with the invention, a suitable flux powder is onewhich has the property of achieving equilibrium very quickly and thenundergoing no further change in its lubrication behavior.

In order, for example, to reduce the reaction rate of the SiO₂ reductionby the Al in the steel, the flux powder used in accordance with theinvention has a high content of Al₂O₃ of >10%. Alternatively oradditionally, to have more SiO₂ available in the equilibrium state, theSiO₂ content of the flux powder is increased sufficiently to obtain abasicity (CaO/SiO₂ ratio) of 0.5-0.7.

Since MnO₂ is more readily reduced than SiO₂ by the Al in the steel, andthus the SiO₂ is protected from this reduction (protected from loss), anadditional measure that can be taken in accordance with the invention isthe addition of MnO₂ to the flux powder.

In accordance with the invention, it is also possible to add TiO₂ to theflux powder as a partial replacement of the SiO₂, since TiO₂ also has avitrifying effect but is not attacked (reduced) by the Al in the steel.

Finally, it is also possible to lower the viscosity of the flux powderin the mold. This increases the consumption of flux powder, and more ofthe Al₂O₃ that is formed is removed, so that an equilibrium with lowerAl₂O₃ concentrations is established. This reduction in viscosity isachieved by the addition of B₂O₃ (borate), Na₂O, and/or LiO₂ to the fluxpowder.

The process diagram of an installation of the invention for producinghot-rolled strip is shown in the schematic drawing and is explained indetail below.

The installation is basically a well-known CSP installation, in which,in accordance with the invention, the distances between the individualinstallation units were changed in such a way that the method of theinvention can be carried out with the requirements that temperatureequalization is brought about in a continuous-type intermediate furnaceimmediately after the solidification and that the slab is thenimmediately hot rolled without intermediate cooling.

Accordingly, the installation illustrated in the drawing consists of athin-slab casting machine 1 and a downstream intermediate furnace 4,into which the slab 3 is fed after it has solidified and has been cutfrom the endless strand 2. The intermediate furnace 4 is followed by arolling mill 5, in which the slab 3, after it has been subjected totemperature equalization in the intermediate furnace 4, is immediately(i.e., without intermediate cooling) rolled out into hot-rolled strip 6.

1. Method for producing hot-rolled strip (6) from a highlycold-workable, high-strength, austenitic light gauge steel withincreased contents of manganese (Mn), aluminum (Al), and silicon (Si)with TWIP properties (Twinning-Induced Plasticity), wherein the lightgauge steel is first cast into an endless strand (2) in a continuouscasting installation (1), cut into slabs (3), and then rolled to thefinal thickness, wherein successive process steps are used, in which alight gauge steel with the given chemical composition of 15-27% Mn, 1-6%Al, 1-6% Si, ≦0.8% C, with the remainder consisting of Fe and impuritiesis cast on a thin-slab casting machine (1) (d≦120 mm) with the use offlux powders and cut into slabs (3), such that suitable mineralsubstances are added to the flux powders to reduce the reaction rate ofthe SiO₂ reduction by the Al in the steel and/or to reduce the amount ofAl₂O₃ that is formed by lowering the viscosity in the mold, immediatelyafter the solidification of the endless strand (2) and cutting of a slab(3), temperature equalization is brought about in a continuous-typeintermediate furnace (4), and then the slab (3) is immediately hotrolled without intermediate cooling.
 2. Method in accordance with claim1, wherein the flux powder has an increased Al₂O₃ content of >10%. 3.Method in accordance with claim 1, wherein the flux powder has anincreased SiO₂ content with a basicity (CaO/SiO₂ ratio) of 0.5-0.7. 4.Method in accordance with claim 1, wherein the flux powder contains MnO₂and/or TiO₂.
 5. Method in accordance with claim 1, wherein, to lower theviscosity of the flux powder in the mold, the flux powder contains B₂O₃(borate), Na₂O, and/or LiO₂.
 6. Method in accordance with claim 1,wherein the intermediate furnace (4) is a roller hearth furnace. 7.Installation for producing hot-rolled strip from a highly cold-workable,high-strength, austenitic light gauge steel with increased contents ofmanganese (Mn), aluminum (Al), and silicon (Si) with TWIP properties(Twinning-Induced Plasticity) for carrying out the method in accordancewith claim 1, which consists of a CSP installation (Compact StripProduction) with the successively arranged installation units thin-slabcasting machine (1), intermediate furnace (4), and hot rolling mill (5),wherein the distances between the individual installation units arechanged in such a way that immediately after the solidification of theendless strand (2), temperature equalization of a cut slab (3) isbrought about in a continuous-type intermediate furnace (4), and thenthe slab (3) is immediately hot rolled without intermediate cooling.